The invention relates to a periodically operating adsorption cooling apparatus with a buffer reservoir and a method for its operation.
Adsorption cooling apparatuses are devices in which a solid adsorbent adsorbs a vapor phase working medium with release of heat at an intermediate temperature (adsorption phase). In the process, the working medium evaporates in an evaporator with uptake of heat at a lower temperature. After the adsorption phase, the working medium can again be desorbed by the addition of heat at a higher temperature level (desorption phase). In the process, the working medium evaporates from the adsorbent and flows into a condenser. In the condenser, the working medium is condensed again and then evaporated anew in the evaporator.
Adsorption cooling apparatuses with solid adsorbents are known from EP 0 368 111 and DE-OS 34 25 419. In the process, adsorbent containers, filled with adsorbents, draw in working medium vapor which is produced in an evaporator, and they adsorb it in the adsorbent filling with release of heat. The adsorption heat in the process must be removed from the adsorbent filling. The cooling apparatuses can be used in thermally insulated containers for cooling and keeping warm foodstuffs. These cooling apparatuses contain a shutoff device between the evaporator and the adsorbent. This allows evaporation and adsorption of the working medium at a later time.
The adsorption cooling apparatus which is known from EP 0 368 111 consists of a transportable cooling unit and a stationary charging station that can be separated from the cooling unit. The cooling unit contains an adsorption container, which is filled with a solid adsorbent, and an evaporator with a liquid working medium. Here, too, the evaporator and the adsorption container are connected by a vapor line that can be shut off. Liquid media, which are cooled to the desired temperature level by the temperature-regulated opening and closing of the shutoff device, flow through a heat exchanger embedded in the evaporator. After the adsorbent has become saturated with working medium, it can be heated in the charging station. The working medium vapor that flows out in the process is condensed again in the evaporator. The condensation heat is removed via the cooling water that flows through the embedded heat exchanger.
The shutoff devices serve, on the one hand, the function of uncoupling the evaporator from the remaining cooling apparatus during the desorption phase, in order to prevent hot working medium vapor from flowing into the cold evaporator, and on the other hand, they serve the function of regulating the cold production of the adsorption phase in the evaporator or shifting it to a later time. Without a shutoff device, the evaporator always becomes hot during the desorption phase, and thus the medium to be cooled that is in contact with it becomes warm.
The problem of the present invention is to protect, in an adsorption cooling apparatus without a shutoff device the medium to be cooled in the desorption phase from impermissible heating.
This problem is solved by an adsorption cooling apparatus of the type having an intermittently heated adsorbent container containing an adsorbent that exothermically adsorbs a working medium during an adsorption phase and with addition of heat again desorbs during a subsequent desorption phase at higher temperatures and with a condenser that leads condensed working medium through a connection line to the evaporator which is in turn connected with the adsorbent through a working medium vapor line and which takes up heat from the medium to be cooled during the adsorption phase, wherein the condenser is coupled to a buffer reservoir that buffers at least a part of the condensation heat of the working medium vapor and that can again dissipate the stored heat into the surroundings even during the adsorption phase.
The coupling of the condenser to a buffer reservoir allows a distinctly more rapid desorption and thus a higher desorption capacity, because the condensation heat, for example, can be removed more effectively due to the starting of convection. The desorption phase can thus be distinctly shortened compared to the adsorption phase. The medium to be cooled is exposed for a shorter period to the high condensation temperatures. In a buffer reservoir of appropriate dimensions, the desorption phase can be reduced to a few minutes, while the adsorption phase can last several hours to several days. During this long adsorption phase, the buffer reservoir can slowly dissipate through small heat exchange surfaces the heat load that was absorbed at high power.
In principle, one can use as a buffer reservoir any of the reservoir media known from the heat reservoir industry, such as liquids, phase change materials (PCM), and solids. Water is cost-effective, and it also allows a high heat transfer capacity. In the process, the condenser can be integrated directly into a water reservoir. The buffered heat is then removed through the external surface of the tank during the slow adsorption phase without an additional heat exchanger, and released into the surrounding air.
It is particularly advantageous for the evaporator to be arranged, with reference to the medium to be cooled, in such a manner that it releases relatively little heat during the desorption phase. This is achieved, for example, by allowing a relatively small amount of medium to be cooled to be in contact with the evaporator, or by omitting any circulation during the desorption phase. If the medium to be cooled is gaseous, as is the case, for example, in refrigerators, it is advantageous for the evaporator to be placed under the ceiling of the refrigerator. Because hot air is lighter than cold air, the cold air mass remains in the lower portion of the refrigerator, while only the air quantity surrounding the evaporator becomes warm. Any goods stored in the refrigerator then remain cold during the relatively short desorption phase. This effect can be further increased by cold storing media and/or radiation screens that are arranged under the evaporator.
For high desorption capacities, high heat conductivity in the adsorbent and good heat transfer from the source of heat are required. It can be particularly advantageous for the heat capacity of the adsorbent, during the desorption phase, to be substantially greater than the heat losses to the surroundings. In this case, one can omit thermal insulation on the adsorbent container casing facing the surroundings. The adsorption heat is then released through the casing during the adsorption phase without additional measures.
It is particularly advantageous to use the adsorption pair zeolite/water. Zeolite is a crystalline mineral which consists of a regular lattice structure made of silicon and aluminum oxides. This lattice structure contains small vacancies in which water molecules can be adsorbed with the release of heat. Within the lattice, the water molecules are exposed to strong field forces that bind the molecules in the lattice in a liquid-like phase. The strength of the binding forces which act on the water molecule is a function of the preadsorbed quantity of water and the temperature of the zeolites. For practical use, up to 25 g of water can be adsorbed per 100 g of zeolite. Zeolites are solid substances without troublesome heat expansion during the adsorption or desorption reaction. The lattice is freely accessible from all sides to the water vapor molecules. The apparatuses are therefore operational in any position.
The use of water as working medium makes it possible to reduce the required regulation effort to a minimum. During the evaporation of water under a vacuum, the water surface cools to 0xc2x0 C. and, during continued evaporation, it freezes to ice. The ice layer can be used advantageously for regulating the temperature of the medium to be cooled. If there is little addition of heat, the ice layer grows, whereas, if a large amount of heat is added, it disappears as a result of melting. Due to the formation of ice, heat transfer is reduced from the medium to be cooled into the evaporator, so that the medium cannot be cooled below 0xc2x0 C. If evaporation is continued, the entire water reserve can turn to ice in the evaporator. The sublimation temperature of the ice layer then decreases to less than 0xc2x0 C.
It is also possible to add substances that lower the freezing point to the aqueous evaporator contents, if the temperature of the medium to be cooled is to be lowered below 0xc2x0 C.
Other adsorbent pairs can also be used where the adsorbent is solid and remains solid even during the adsorption reaction. Solid adsorbents have low heat conductivity and limited heat transfer. Because the heat transfer from the adsorbent container to the surrounding air that takes up heat is of the same order of magnitude, heat exchangers without ribs are recommended in principle, for example, plates, pipes and corrugated metal tubes. Some solid adsorbents, such as zeolites, are sufficiently sturdy to compensate even for external excess pressures on thin-walled heat exchanger surfaces. Additional reinforcements or thick-walled heat exchanger surfaces are therefore not necessary.
Moreover, solid adsorbents can be processed into molded bodies. A single molded body, or a few molded bodies, can form a complete cost-effective adsorbent filling.
For an economical operating procedure with zeolite/water systems, it is recommended to use desorption final temperatures of 200-300xc2x0 C. and adsorption final temperatures of 40-80xc2x0 C. Because zeolite granulates have a particularly low heat conductivity, the adsorbent containers should be designed so that the heat conductivity path for the processed quantities of heat does not exceed 3 cm.
As a heat source for the desorption phase, any of the known devices are suitable, provided that the temperature level required for the desorption reaction is achieved with them. Electrically heated plates or cartridges whose geometry is adapted to the adsorbent container are advantageous. Also suitable are heating devices which heat the adsorbent filling by radiation or induction (eddy current). The heating surface used during heating of the adsorbent with a flame can also be used as a heat exchanger surface for heat release during the adsorption phase. It is thus possible to omit the conventional double installation of heat exchanger surfaces.
It can also be advantageous to adapt the geometry of the adsorbent container specifically to heat release during the adsorption phase. In the case of heat release into the surrounding air, it is preferred to use large heat exchanger surfaces that promote flow.
The working medium condenses predominantly in the condenser. The condensate must be led from there to the evaporator. If the adsorption cooling apparatus is simply constructed, then the condensate must be able to flow back into the evaporator without additional help. This is always easy to achieve if the condenser and thus also the heat buffer are in a higher position than the evaporator. The condensate can then surely flow back during the desorption phase by gravity. In cooling apparatuses where this is not possible, it can be advantageous for the condensate to be stored in the condenser or in a collection tank, to be drawn upward into the evaporator at the beginning of the adsorption phase, when the vapor pressure in the evaporator decreases.
Expensive electronic regulation must be omitted in cost-effective cooling apparatuses. However, since adsorption apparatuses necessarily produce a highly variable cooling power, it is advantageous for the cooling power to be limited in a simple manner. According to the invention, the cross section of the working medium vapor line to the adsorbent is decreased for that purpose. This can also be achieved, for example, by expansion elements that decrease the cross section of the pipe to the adsorbent with decreasing temperature. Bimetal elements that are incorporated in the evaporator are a particularly cost-effective way of narrowing the outlet of the evaporator with decreasing evaporation temperatures.
Because the evaporator, as a function of the system, is raised to the termperature level of condensation with each desorption, and because, at the beginning of the adsorption phase, it must again be cooled to the lower temperature level of evaporation by evaporation of a portion of the working medium, it is advantageous to keep the thermal mass of the evaporator low and to set the quantity of liquid working medium such that, to the extent possible, the entire working medium is evaporated at the end of the adsorption phase. Toward the end of adsorption, the quantity of working medium in the evaporator becomes increasingly smaller, and, consequently the wetting of the heat exchanger surface to allow the uptake of heat from the medium to be cooled becomes increasingly more difficult. According to the invention, the evaporator contains wetting agents for this operational state that distribute the remaining working medium homogeneously over the internal evaporation surface. For this purpose, nonwoven glass fiber materials have been shown to be satisfactory; they are applied as a thin layer on the corresponding evaporator surfaces.
A preferred form of the adsorption cooling apparatus according to the present invention, as well as other embodiments, objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments thereof, which is to be read in conjunction with the accompanying drawings.